George Rogers, president of Printer's Edge Ltd., identifies heat generation, tearing, and material buildup as some of the challenges associated with the use of belt grinders. "You develop a lot of friction and heat because you're trying to sand a pliable material," he explains. "Urethane rubber, being an elastomer, is going to give, and when you push it up against the wheel or belt, it deflects at various levels depending on how much pressure is put on. A tearing effect causes little peaks and valleys on the edge of the squeegee blade. Those peaks will break off, and the valleys tend to cause streaks in the prints."

Ground urethane can become lodged in the grit of the grinding belt. When enough squeegee material builds up on the belt, the machine may sharpen blades erratically or ineffectively. Rogers says lack of a uniform grind across the blade and a reduction in material removal are tell-tale signs of urethane buildup on the belt. But he also points out a noteworthy advantage to using belt grinders: Operators can build fixtures to hold squeegee blades at different angles. The use of such jigs enable the grinder to produce custom blade profiles and edge characteristics.

The short-belt grinder (Figure 2) also uses a grinding belt and pulleys or wheels as drivers, but the squeegee is held still while the belt-driving system travels from one end of the squeegee blade to the other. The single point of contact made between the moving belt and the squeegee blade's surface puts less pressure on the squeegee, which means a reduction in blade deformation during grinding. Edge straightness and uniformity of height also improve when the holding device and wheel movement are held parallel. Rogers says short-belt grinders are a better alternative to long-belt machines, but heat and buildup on the belt can still lead to problems.

"The wheel type of sharpeners are faster, more consistent, and easier for most screen printers to use," says David Landesman, co-president of Lawson Screen & Digital Products, Inc. This sort of grinder may feature a stone wheel, diamond wheel, or an abrasive belt wrapped around a wheel.